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CHAPTER VII. THE FILM
(1) The 35 millimeter-wide motion picture film is usually nitro-cellulose (inflammable), though cellulose acetate (slow-burning) is available. The two are exactly the same, except for their ingredients. (2) The 35 mm. film is 1.376 to 1.378 (practically l}i inch) wide. (3) Its various dimensions, as approved by the Society of Motion Picture Engineers, may be examined in Fig. 54.
(4) Celluloid, from which films are made, is manufactured in wide sheets approximately 2,000 feet in length. These sheets are coated with photographic emulsion, some negative and others positive, according to their purpose. They are then split into ribbons 35 mm. wide and perforated. The combined thickness of the film stock and the photographic emulsion is from five and one-half to six thousandths of an inch. The photographic emulsion is about 0.001 of an inch thick.
(5) The camera impresses a series of snap-shot photographs upon a negative film at the rate of 24 per second. (6) The over-all dimensions of these "snapshots" are 0.631 inch by 0.838 inch. The chief difference between the negative impression and the photograph on the positive print is that everything is in reverse, the clear whites in the negative will be opaque blacks in the positive, and opaque blacks in the negative will be clear white in the positive print, with graduated shading of tone. (7) In both negative and positive, each photograph and its surrounding dividing line (known as the "frame line") occupies precisely 0.75 (24) of an inch, or 16 "frames" to each foot of film.
(8) Today the dimensions of the motion picture projector aperture, as approved by the Society of Motion Picture Engineers, is .600 by .825 of an inch. Note that
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